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Post by joanlluch on Sept 29, 2014 13:55:54 GMT
Julian, for the valve gear design I am using the recommendations on "Design Procedures for Walschaerts' and Stephensons's valve gears" by Don Ashton. I think it is a good book and I learned quite a bit from it. As you know I have also drawn the valve and simulated it on a 3D Cad computer program, so I can clearly see what is going on for all cut-offs. Of course I am not an expert, and I never ran a loco, but that's already the first thing I stated. I am sure that experience and knowledge will come with time, as has happened with any new venture I have started in the past.
I will assist next month to a local event where live steam locos will be shown/run and I expect to meet as well interesting people there.
I know your recommendation is to build a "known" design first, but that's simply something I am not stimulated to do. I definitely want to draw my own design, for the good or for the bad.
Also, I am not implying that much can be improved, but maybe there are a couple of things that have been done in a particular way for decades that can be revised. Among others, I would like to investigate if a loco could be build entirely (or mostly) directly from drawings by CNC equipment, or if we could use maintenance free bearings (no lubrication required) for the moving parts of the loco. Or if some of the materials currently being used can be replaced by more suitable ones, such as PTFE rings in the pistons.
As well, there may be subjects that are perceived or not as improvements, depending on who you ask. Believe it or not, something that annoys me is the ubiquitous use of imperial units on the existing designs. So having something designed in metrical units is already a significant improvement from my point of view. I strongly believe that this alone is setting back a lot of possible new hobbyists.
In any case, it is completely right that the experiences of others are crucial when you start something new, and for this reason I really appreciate your comments and suggestions.
John
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Post by Roger on Sept 29, 2014 14:06:11 GMT
It's well worth looking through all the threads on this forum with regard to bearing materials and seals. You're not the first to question the suitability of certain materials but you will see that many such as Vesconite and Peek have been tried. Some combinations are surprising, such as soft Silver Steel against Hardened Silver Steel. I'm sure you will achieve much with what you already know, but to ignore the vast amount of knowledge here on such things is probably a mistake. Most things have been tried and it's always worth asking the question before you go ahead and decide on something. I was convinced that using miniature ball bearings on the valve gear would be a good idea. By all accounts it's not. It's hard to second guess what the answers to these questions are going to be. Many people have used PTFE piston rings and similar for piston valves. Again, I'd urge you to research the Forums to save reinventing the wheel. I wish you every success with your venture, it's going to be a fascinating project that I hope you'll share with us.
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Post by joanlluch on Sept 29, 2014 14:38:58 GMT
Thanks Roger. That's definitely what I will do. I found this forum just several days ago, so I need more time. About PTFE I already found some interesting things in the forum after performing a search two days ago. Someone described his experience and was good overall, but I still need to figure out if it is reliable on the long term. It's interesting your comment on ball bearings. I just made a search on the forum and I think I found what you refer to. I wonder if someone has tried this instead for bearings: www.igus.co.uk/iglidur. They claim to be lubrication free and some of them support high temperatures. John
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Post by joanlluch on Sept 29, 2014 14:52:33 GMT
Hi Roger, me again. I just figured out that your mention of Vesconite was just the kind of material I was thinking about. It's just I came across a different supplier.
According to people shared experience in this forums that kind of bearings work well, don't they?
John
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uuu
Elder Statesman
your message here...
Posts: 2,810
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Post by uuu on Sept 29, 2014 16:33:05 GMT
There's a growing body of evidence based on experimentation with various plastics - Vesconite for bearings, PTFE for piston rings etc. The published loco designs usually don't specify these materials, so it's down to brave individuals to depart from the design and have a go.
Apologies for not taking you fully seriously at the beginning. We get several "I'm new to playing the piano and Steinways are so expensive, I thought I'd make my own. Where do I start?"
Wilf
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Post by Roger on Sept 29, 2014 17:04:35 GMT
Hi John, The last thing I want to do is to put you off any experimentation, there's still plenty of scope. There's not enough evidence in my opinion to wholeheartedly say that advanced plastics are a better option in places than traditional ones. It think it very much depends on where you elect to try them. I've made some Vesconite ones for the big end of the main connecting rods as an experiment. I can see them being good where the stresses aren't too great and where rigidity in the bearing isn't a big issue. That probably means that it's going to be ok for the connecting rods but not the small end of the main connecting rod which gets a real battering! I also can't see it being much use in the miniature pinned joints of the valve gear because it would introduce too much play. I also wouldn't use it for the lifting shaft for the valve gear which also needs to be rigid.
It would appear that PTFE piston and piston valve rings have a very long life and there are several schools of thought about whether they should be split or solid and what thickness they should be. There are also differences of opinion about whether they should have a backing ring of some other material to provide the outward pressure. Some people are using simple Silicone 'O' rings for pistons with success too. Other 'O' ring materials don't appear to stand up well though. I'm going to fix 'Quad ring' seals instead of 'O' rings on water pumps. I'm also using 'O' rings in preference to other sealing methods where possible. I too am making an all Metric locomotive and have found sources of appropriate nuts and bolts through the help of members here. If you'd like to PM me, with your email address I'll let you have the information that I've gathered and also a handy inch/metric converter spreadsheet that makes converting any imperial drawings to metric a lot more convenient.
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Post by vulcanbomber on Sept 29, 2014 17:32:48 GMT
. I am using mm in all dimensions. I chose the Baker valve gear because It has no sliding parts and thus it should be possible to use lubricant free polymer based bearings. My question was whether it is worth to bother with implementing internal cylinders (like the Britannia) or not. I am happy the consensus has been "2 cylinders is perfectly fine" because, as you (and others) recommended, It is best to keep things as simple as possible. The Standard Class 7's (Britannia) we're 2 cylinder engines, the only Standard Engine with more than 2 Cylinders is the Duke
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Post by donashton on Sept 29, 2014 19:24:49 GMT
I can't help feeling that the place is getting buried in plastics for the wrong reasons. The purpose of the valve gear is to provide equal power from each end of the double-acting cylinder and that should dictate the choice. Even in full size there were common mistakes right up to the last. Britannia did not achieve better in this respect than 30 years earlier, though E.S.Cox quite rightly pointed out to those criticising plain bushes in valve gears that simple and cheap renewal at 55,000 miles would take some beating.
Where authenticity is not an issue Stephenson's gear is head and shoulders above any other in terms of duty and purpose and has no wearing difficulties greater than any other gear. Full size trends were misguided.
Don.
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Post by joanlluch on Sept 29, 2014 22:17:26 GMT
wilf, No problem. You can believe that since English is not my first language it's hard to me (or impossible) to catch some subtleness of the language implying sarcasm, jokes or the like. Also it is quite common by natives in any language to confuse lack of fluency or grammar errors by poor intelligence or skills. This happens all the time. If you ever tried to learn a foreign language you will know how difficult it is to master. English is my 4th language actually, so not the one I am more comfortable with, unfortunately. roger, do you imply that these plastic materials are "flexible", or "not rigid" so that they may present gaps as parts are moving? About PTFE, now I can think on just doing the opposite. i.e. the piston made of mechanised metal (such as gunmetal or stainless steel) but then have the cylinder internally covered by plastic to the right diameter. That's possibly a foolish thought, but who knows? donashton. What is your opinion of the Baker valve gear? After I simulated it on a 3D CAD I realised that events on each side of the valve are not fully symmetrical because basically the upward movement of the bell crank induced by the radius rod is faster towards the upper end than towards the lower end. This happens both in forward and reverse motion. I can think on ways to minimise it, but I don't see them applied on photographs of the mechanism I found published on the internet, or am I missing something? Not sure if you are the same Don Ashton as the Author of "Design procedures for ... valve gears" book, but anyway that book is pretty informative and useful. John
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Sept 29, 2014 22:30:37 GMT
hi john,
wilf has a very dry wit that ive known for some 25 years and always makes me smile - i accept that you might not pick up on these witticisms!
vesconite has been proven on fullsize coupling rod bearings, and extensive use has been made of PTFE in miniature loco work since the early 1980s.
don ashton is indeed the author of the books you quote, and we all owe a huge debt to don for his expert generous help on valve gears. ive built 3 locos with valve gear to don's books, and don has very kindly re-designed for me the valve gear for 2 other locos ive built/am building.
i think you need to firm up on what you intend to do re your first loco then perhaps we can help more specifically. ive 31 years experience at this game - don has many many more years as do many other very valued and esteemed members of this forum!
cheers, julian
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Post by Roger on Sept 29, 2014 23:37:04 GMT
Yes, I do mean that these plastics are not as rigid as metals and they also usually require more clearance. If the precision of the bearing location isn't vital then this probably doesn't matter. For things like valve gear, I don't think they are suitable because they introduce clearances and flexibility that compromise the valve events. I like your lateral thinking about applying bearing material to cylinder bores. All these ideas are worth airing. It's easy to try to think of novel solutions to problems that don't exist though. Cylinder wear isn't an issue from the little I know but small end bearings on connecting rods is. The trick is to apply new techniques where the old ones fall short and forget about the other ones. There are some bearing combinations that need to be avoided and they are not always obvious. Stainless steel can be a very poor choice in some places, it can pick up and cause damage. Julian will tell you what to avoid. Some things seem like a great idea but just don't work for one reason or another.
Wow! Four languages! Good grief, I have enough trouble with one!
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Post by joanlluch on Sept 30, 2014 6:34:15 GMT
Hi Roger, I meant using PTFE in the cylinder, not a bearing material such as Vescolite. I understand that cylinder wear is not an issue provided it is well lubricated. PTFE is actually a soft material but supports high temperatures and has low friction. It potentially is able to create good "sealing" conditions as In the industry it is used to make joints. As PTFE has already been tried on pistons (both valve piston and main piston) my idea was using it in the cylinder as opposed to the piston. Although of course I have no idea of whether this is a good idea or not, because one possible problem is that PTFE expands with humidity (which is in turn the reason why it is a good material for joints).
About stainless steel my experience is that when it is subjected to high combined load and friction it "peels" instead of just "wear". So the material tends to produce some sort of tiny metal balls or scales in the surface that in turn increase friction and produce yet more scales. Once this starts to develop the outcome is a disaster, but the good point is that compared with softer materials a relatively strong load and friction is needed on stainless steel before that happens. Also stainless steel combines really well with plastic materials because essentially its surface can be really polished and bright and it will always win against the plastic, I mean it is the plastic that experiences the wear.
Thanks for your comment about my 4th languages, Actually the first 3 were not that hard given where I live. They are Catalan (native), Spanish (60% population speaks it natively in my country), French (I live just a few Km of France and you know frontiers are no longer there, about 10% population speaks it to some extent). And finally English; the only one I have been taught in language academies (also in the UK), as virtually nobody above 40 years old (not to mention 50 y.o.) is able to go much beyond "Good Morning, my name is..." in my country.
Julian, I think I already know what I want to do and what steps to follow. I guess I seem to be wandering around because I am at the very early stages and am asking several disparate things. I will try to be more specific on my questions in the future and separate them on different threads. Actually the single initial question I asked was whether it was worth to have 3 cylinders as opposed to only 2 on a free lance design. That was answered.
John
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Post by Roger on Sept 30, 2014 7:31:42 GMT
Hi John, I think the conditions in the cylinder, especially with superheating, make it a very demanding environment for plastics. As you mention, PTFE does expand with humidity and I think that's the same to some extent with all of the 'O' ring materials. You'll find that a lot of Model Engineers swear by the use of cast iron for all of the cylinder parts but corrosion is an ever present danger when laying up the locomotive for any length of time. That alone puts me off those materials. I think there's scope for experimentation on piston rod seals, I'm trying to see if there's a source of Silicone 'Quad Rings', I don't think Nitrile ones are going to be suitable. I don't know what the favourite material is for the rods, but I suppose there's no reason why there couldn't be a small insert of Vesconite or similar where the rod comes through the cover. Stainless Steel has been tried for piston valves with the results you describe, it's not a good idea. Piston valves made from solid plastics have also been tried but they expand too much. Solid metal ones with PTFE rings seem to give the best of both worlds.
Do bear in mind that I have no experience in any of this, I'm merely echoing what I've read on the Forum and elsewhere while I've been doing my research into SPEEDY. Some members have been at this game for 30 years and it's well worth giving their suggestions long and careful consideration. Unfortunately, they don't all agree, but that just makes it more interesting in my opinion. If you read my thread about SPEEDY you will see the many and varying opinions on everything from tyre tread profiles, lubricators, bearing materials and a host of other things. So long as you don't stray too far from tried and tested things, you'll end up with a locomotive that works even if it's not the best.
I'm afraid the British don't usually take speaking foreign languages seriously because English is most people's second language. I can scrape by with a little schoolboy French and German but my vocabulary is minuscule. I'd love to be able to speak these but I don't have the mind for it.
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Post by joanlluch on Sept 30, 2014 7:52:10 GMT
Hi Roger, I very much like your involvement and thorough replies. It's a pleasure to read them. I am definitely following your advice and reading as much as I can from others experiences, including of course your SPEEDY thread. I'm still waiting some Britannia drawings from BlackGates that were reportedly shipped one week ago. I hope they have not been lost in some post office. I begin to be worried about that possibility. The drawings should give me an orientation on the right dimensions for several parts and they should allow me to continue my 3D drawings. I will keep in touch. John
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Post by Roger on Sept 30, 2014 8:14:33 GMT
Hi John, It's interesting to hear your ideas, I have a few of my own I'm trying out. I'm relatively new to this although I dabbled as a teenager before giving up. With a lifetime in Engineering and finally a suitable workshop, I thought it was time to revisit this hobby and it's been really rewarding. The members on this Forum have been so generous and patient, it's made the whole experience so much more pleasurable. With your own skills and experience, we all stand to learn something from what you're doing, it's very much a two way street. It's an amazing resource and there are some great characters here. Once they've established that you're serious and capable, you'll find the advice flowing freely.
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Post by donashton on Sept 30, 2014 18:37:22 GMT
Hi John,
Forgive a rather late answer. This day has been a very busy one solving problems in both models and full size. At 75 I should have been pottering about in the garden on such a warm sunny day, instead of bashing my poor hearing against riveting and hiding my poor sight from welding flashes. Wouldn't be quite so exciting, of course.
Baker valve gear was invented for traction engines where duties are different from locomotives. It was marketed privately by the Pilliod company, as opposed to being available to industry. It's lack of popularity compared to 'market leaders' Stephenson's and Walscherts' may be due to having to pay fees to use it or simply because it showed no improvements.
All the equality problems in the double-acting cylinder have the conversion of linear to rotating motion at the root. The move over to Walschaerts' gear during the 20th century does not reflect any superiority. In fact Stephenson's gear properly designed has no peers. In it's best form the angularity problems are equally opposed to produce almost immaculate valve events. In other words, errors are balanced against each other instead of trying to ameliorate by requiring long rods and small circles. Builders faithfully adhering to prototype don't have an option here, but the freelancer can engage the best of at least 100 gear inventions unless there is an impelling reason not to do so.
If my book has helped you I am glad. Some readers find it a little hard-going but amongst the several thousand that have been distributed all over the globe some benefits may have arisen. It is worth noting that even a career draughtsman in the design office may only have had to design a gear from scratch perhaps once or twice in a lifetime. No wonder there have been so many mistakes in reality.
Best wishes,
Don.
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Oct 1, 2014 13:21:12 GMT
John
Nobody has really answered your question regarding more than two cylinders.
Two cylinders are all a locomotive needs, they give four power strokes per revolution and no dead spots. These cylinders can be mounted outside the frame with all working parts exposed for maintenance. This factor was considered important for modern locomotives as for the BR standard designs. Inside cylinders and drive/valve gear increases the difficulty, read cost and reliability, of maintenance. The drawbacks of two cylinders are problems with dynamic balancing and the width at platform level imposed by the large cylinders.
Where more cylinder capacity is needed then designers have had to resort to three or four cylinders at the cost of increased complexity and maintenance requirements.
In Australia there were only(excluding Beyer Garratts) four classes of locos with more than two cylinders, 57 & 58 classes of NSWGR and S & H classes of the VR. On the South Australian Railways locos of the same tractive effort, 500 and 600 classes had two cylinders because the loading gauge was more generous allowing bigger cylinders.
Regards Ian
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Oct 1, 2014 13:29:44 GMT
John
Regarding Baker gear. It had wide acceptance in the USA but I have not heard of it being used in the Uk. There is one loco preserved in Australia which has Baker gear, see 621 of Steam Ranger in SA. It is claimed that this gear gives very square events and this loco does have a distinctive crisp bark. It was only ever used on one class of engine and when the later 520 class was designed it was rejected and Walschearts gear was used; this later loco class proved to be much more efficient than its Baker geared predecessor.
Ian
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Post by Deleted on Oct 1, 2014 14:41:39 GMT
QUOTE ---"Two cylinders are all a locomotive needs, they give four power strokes per revolution and no dead spots."............Only when some form of Quartering is used thus avoiding both cylinders being at a "Dead" centre at the same time........Technically you could actually just have the one double-acting cylinder set between the frames onto a crank axle....but stopping at a dead centre would be a problem--- you'd need to bar the crank over-centre before continuing-----------------something most single-cylindered Traction Engine drivers have to do now and then........
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Post by joanlluch on Oct 1, 2014 15:21:43 GMT
Hi Don. I very much appreciate your reply. The book certainly advances on some aspects that are sometimes hard to grasp by the novice. In my case it has been very useful because it demystified for me the whys and the hows or valve gears.
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